1. Field of the Invention
The present invention relates to flow cytometry, and more specifically, it relates to flow-cytometric biological, medical and other assays.
2. Description of Related Art
In previous Flow Cytometers (FCM), the right-angle-scatter (RAS) light has been viewed perpendicularly to the liquid flow, typically using a high NA microscope objective lens or fiber optic. Some of the difficulties associated with this approach are the very limited depth of field of high numerical aperture (NA) lenses, typically only a few .mu.m, and the necessity to align precisely the exact focal point of the lens with that of the excitation light source. In addition, an obscuration bar is typically needed to block the laser light that has been scattered from the flow stream surface. As is shown in FIG. 1, a NA 0.6 lens captures less than 14% of all solid angles. U.S. Pat. No. 5,475,487, incorporated herein by reference, discloses the use of the unconfined aqueous flow stream itself as an index-guided optical waveguide, which captures approximately 17% of the solid angles. This very modest increase is augmented by the fact that all of the scattered light is trapped; there is no "focal point" for this configuration. Alignment simply requires aligning the light source onto the flow stream; no obscuration bar is needed, and the liquid optical waveguide is then automatically "aligned." This approach provides robust, stable light collection.
For the collection of elastically-scattered light, another immense advantage occurs: The background of scattered light is extremely low when using the flow-stream waveguide (FSW), because the same physical properties which confine the desired light within the stream also keep random scattered light out. It has been found that this FSW configuration gives the elastically-scattered RAS signal a much higher signal-to-noise ratio than that of forward scattered light. While this is extremely effective as described, there are applications, such as cell or chromosome sorting through the use of electrostatic deflection plates, which require the formation of a free stream of droplets after the cells or chromosomes have passed through the laser(s). In these cases, the previously patented technique (FSW) will not work because the formation of drops destroys the optical waveguide between the laser-excitation zone and the detector. Similarly, there are applications in which the necessary liquid flow is so slow that instabilities in the stream would also disrupt the optical waveguide between the laser-excitation zone and the detector.